The present invention relates to a resistive element used, e.g., as a position sensor which detects a position of a moving mechanism of various electronic apparatuses, and it also relates to a variable resistor using the resistive element, and a method of manufacturing the resistive element.
Electronic apparatuses have been required downsizing land cost reduction for years. This market situation entails increasing a number of cases where a variable resistor type position-detecting-sensor is desirably used for detecting a moving mechanism of an electronic apparatus. The variable resistor employs a resistive element, and a dc constant voltage is regularly applied across the variable resistor. The position detecting sensor is required to be small, and yet, have a wide effective range.
The conventional resistive element, the variable resistor using the element and a method of manufacturing the element are described with reference to FIG. 9 and FIG. 10. FIG. 9 is a plan view of resistive element 5 employed in a conventional rotary variable resistor. In FIG. 9, insulating board 1 is made of, e.g., phenolic resin. Horseshoe-shaped resistive film 2 is printed on the surface of board 1. Ring-shaped current collector 3 is printed in conductive ink of silver system inside resistive film 2 at a given interval from film 2.
At lower side of terminal sections 2A and 2B of resistive film 2, electrodes 4A and 4B are printed. Printed electrode 4C extends from collector 3 and runs downward between electrodes 4A and 4B.
In order to give the variable resistor a predetermined variable range, printing procedure is regularly arranged as follows: First, collector 3 and electrodes 4A, 4B and 4C are printed simultaneously with good-conductive ink of silver system so that the respective electrodes can be electrically independent with each other. Then resistive film 2 is printed.
FIG. 10 is a schematic drawing of the rotary variable resistor using this resistive element 5. As shown in FIG. 10, electrodes 4A, 4B and 4C of resistive element 5 have respective terminals 6A, 6B and 6C for external use, and sliding contact 7 is integrated into element 5 so that contact 7 can resiliently slide on resistive film 2 as well as collector 3.
The rotary variable resistor using resistive element 5 having the structure discussed above is used as a sensor in the following manner: A dc constant voltage is applied across terminals 6A and 6B, and contact 7 slides on resistive film 2 from first terminal section 2A to second terminal section 2B (electrode 4B), thereby obtaining a desirable output voltage across terminals 6A and 6C.
However, in the conventional variable resistor discussed above, a potential difference is produced between terminals 6A-6C and between terminals 6C-6B when a dc constant voltage is applied for use. In this status, when ambient moisture is high, moisture in the air forms into dew on board 1. Then the silver on the anode side reacts with the water, and an inter-reaction between silver-ion and hydroxide is repeated before the silver travels on the surface of board 1 to the cathode side, where cathodic reduction is performed and the silver is deposited. When the silver deposition progresses, the anode and cathode are finally shorted. This is called xe2x80x9csilver migrationxe2x80x9d, and the conventional variable resistor sometime has encountered this silver migration. A countermeasure against the silver migration is provided, i.e., electrodes 4A, 4B and 4C are desirably arranged with a given space between electrodes 4A-4C and between electrodes 4B-4C.
Since the electronic apparatuses are downsized due to the market requirement, the resistive element used in the variable resistor is also downsized and the spaces between electrodes are narrowed. Further, the sensor discussed above uses the resistive element in more cases, therefore, an improved resolution, i.e., better accuracy of position detection, is required. For this purpose, a wider operating range is required to the resistive element. In other words, the resistive film having narrower spaces between the electrodes disposed on both the terminal sections is required. However, it is difficult for the conventional resistive element to be downsized with a wider operating range and prevent the silver migration simultaneously.
The present invention addresses the problems discussed above, and aims to provide a downsized resistive element which can prevent silver migration when a dc constant voltage is applied for use and accommodate a wide range of rotary angle with ease. The present invention also provides a variable resistor using the downsized resistive element, and a method of manufacturing the element.
The resistive element of the present invention comprises the following elements:
(a) a sheet of resistive film disposed on an insulating board;
(b) a current collector disposed at a given interval from the resistive film; and
(c) electrodes conductive to both the resistive film and the collector.
Slits for splitting the electrodes apart are formed by punching the insulating board. This structure allows the resistive element to maintain the creepage distances between the electrodes because of disposing the slits even if the spaces between the electrodes are narrowed. As a result, silver migration is regulated from occurring and shorts between the electrodes are eliminated. A highly reliable resistive element is thus obtainable.
A method of manufacturing the resistive element of the present invention comprises the following steps:
(a) forming an integrated electrode and a current collector on an insulating board, the integrated electrode including a plurality of electrodes for external use;
(b) forming a sheet of resistive film, at least of which one terminal section overlying on the integrated electrode, and having a given interval from the collector; and
(c) punching the insulating board to form slits at given places.
Step (c) splits the integrated electrode apart and forms a first electrode conductive to the terminal section as well as a second electrode conductive to the collector, both the electrodes being independent with each other electrically.
This method can adopt a printing process and a punching process, both the processes are advantageous for continuous production, which results in volume production at a low cost, in addition to regulating the silver migration and eliminating shorts between the electrodes. The downsized and quality resistive element with high reliability is thus obtainable.